Author Topic: Motorola GP300 Discriminator Output  (Read 3618 times)


  • Newbie
  • Posts: 1
    • View Profile
Motorola GP300 Discriminator Output
« on: 06 Apr 2014, 07:39:25 »
I happen to have several UHF GP300's sitting around. I thought I might be able to turn one into a scanner and want to access discriminator out.

I then began searching for, and finally found, a schematic for the GP300. While searching the schematic, I found what I believed to be adequate discriminator audio output. In the linked pdf, ( one can see PI, pin 7 (and 6 for some reason) appear to have disc. audio out. I soldered an audio cable to pads 7 and 8 (8 is ground) I connected it to my sound card line in.  I scoped it with a computer oscope application, and the signal appears correct, but truthfully, I have really no idea what I am doing with the oscope.

I was able to listen to the audio out through my computer, and can tell when it receives a p25 signal (in addition to the flashing red receive light on the radio) because the amplitude of signal lessens when receiving a digital signal.

I am thinking I might need a variable resister, or perhaps an RC circuit, or perhaps, I just don't have discriminator audio. Then again, it might be something else.

Can anyone advise?